1. Field of the Invention
The invention relates to a process for the preparation of solutions of hardenable urea-formaldehyde resins for impregnating and coating carrier webs to be used in coating the surface of wood-based panels.
2. Description of the Prior Art
German Offenlegungsschrift No. 24 48 472 discloses a process for the preparation of solutions of hardenable urea-formaldehyde resins which are especially useful for impregnating and coating carrier webs to be used in coating the surface of wood-based panels. The process comprises the following steps:
(a) reacting an aqueous solution of urea and formaldehyde in a molar ratio of 1:1.5 to 2.5 and in the presence of 0.2 to 1.0 mmoles of an aminosulfonic acid and 20 to 100 mmoles of ammonia or 5 to 25 mmoles of urotropine (in each case based on 1 mole of urea) for 10 to 30 minutes at temperatures of 70.degree. to 95.degree. C., until the 50% solution has a viscosity of 55 to 65 cps at 20.degree. C., and then
(b) adding 0.8 to 10 mmoles of an aminosulfonic acid to the mixture and maintaining the pH at a value between 4.0 and 4.5 with ammonia during a reaction time of 10 to 25 minutes at 70.degree. to 95.degree. C. until the 50% solution has a viscosity of 80 to 110 cps at 20.degree. C., and finally
(c) adding 40 to 200 mmoles of ammonia or 10 to 50 mmoles of urotropine as well as 0.1 to 0.3 moles of urea to this reaction product and reacting this mixture for 5 to 45 minutes at a temperature of 70.degree. to 95.degree. C., until the 50% solution has a viscosity of 85 to 125 cps at a temperature of 20.degree. C.
Carrier webs impregnated and coated with such urea-formaldehyde resins have proven themselves particularly useful for coating wood-based panels. This is the case especially when wood-based panels are coated in so-called short-cycle presses, i.e., in presses at a pressure of up to 2000 kPa which is about 20 kg/cm.sup.2, using platten temperatures of 160.degree. to 180.degree. C. for a period of ca. 60 to 180 seconds which conditions are effective to achieve the desired coating.
In many cases, especially when the formation of particularly high-grade surfaces is desired, the coating of the wood-based panels takes place in multi-daylight presses at the same pressure, at a temperature of about 130.degree. to 150.degree. C. and during a period of 2 to 6 minutes with subsequent cooling. With this procedure, it was noted that surfaces which are susceptible to cracking are often formed as a result of the longer action of heat.
If the hardener content is reduced in order to eliminate such defects, the danger exists that, under the conditions of the short-cycle presses, the resins will not be fully cured. Accordingly, it was necessary to match the hardening characteristics to the respective hardening conditions by adding a larger amount of hardening catalyst to those impregnated and coated carrier webs which were processed under short-cycle conditions. The manufacturer was therefore forced to keep supplies of carrier webs on hand which had been impregnated and coated with different resins in order to accommodate the requirements of the pressing conditions under which the surfaces are coated.